An electric power steering apparatus (EPS) that energizes a steering system of a vehicle by using a rotational torque of a motor as an assist torque, applies an assist force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque (steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a pulse width modulation (PWM) control.
A general constitution of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage command value Vref obtained by performing a compensation and so on with respect to the calculated current command value.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 30 mainly comprises a CPU (or an MPU or an MCU) , and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel. The calculated current command value Iref1 is inputted into an adding section 32A and is added with a compensation signal CM from a compensating section 34 for improving a current characteristic. The maximum value of the added current command value Iref2 is limited at a current limiting section 33, a current command value Irefm that is limited the maximum current, is inputted into a subtracting section 32B, and a motor current detected-value Im is subtracted from the current command value Irefm.
A deviation I (=Irefm−Im) being a subtraction result at the subtracting section 32 is inputted into a PI-control section 35, the PI-controlled voltage command value Vref is inputted into a PWM-control section 36 and is calculated duty ratios in synchronous with a carrier signal CF, and the motor 20 is PWM-driven through an inverter 37 with PWM-signals. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B.
The compensating section 34 adds a self-aligning torque (SAT) 343 detected or estimated with an inertia 342 at an adding section 344, further adds the result of addition performed at the adding section 344 with a convergence 341 at an adding section 345, and then outputs the result of addition performed at the adding section 345 as the compensation signal CM thereby to improve the characteristic of the current command value. In addition to the compensating section 34, although there are many cases to provide a vibration-damping control section to suppress the vibration of the steering wheel, these the compensating section and the vibration-damping control section are not indispensable.
According to an exemplary embodiment of an electric power steering apparatus, if a great assist torque is applied by the motor near the maximum steering angle (rack end) of the steering system, a great shock occurs when the steering system reaches at the maximum steering angle and the hitting sound is generated. In this way, a driver feels uncomfortable, and there is a possibility to lock the steering system including the motor.
In this connection, Japanese Published Unexamined Patent Application No. S61-184171 (Patent Document 1) discloses the electric power steering apparatus to decrease the assist torque of the motor when the steering angle is equal to or more than a predetermined angle or to interrupt the application of the assist torque. Patent Document 1 also discloses the electric power steering apparatus not to occur the assist torque at the time.
Further, Japanese Published Unexamined Patent Application No. 2001-253356 (Patent Document 2) discloses the electric power steering apparatus provided an unloader correcting section to correct the steering torque of the motor corresponding to the steering velocity when the steering angle exceeds a predetermined angle near the maximum angle.